Ducted fan gas turbine engines for powering aircraft conventionally comprise a core engine which drives a propulsive fan. The fan, in turn, comprises a number of radially extending aerofoil blades mounted on a common hub and enclosed within a generally cylindrical casing.
There is a remote possibility with such engines that part or all of one or more of the fan blades could become detached from the remainder of the fan. This might be as the result of, for instance, the engine ingesting a large foreign body such as a bird. In the event of this happening, it is extremely important that the detached blade or blade portion is contained by the fan casing. Thus the fan casing must be sufficiently strong to ensure that the detached blade or blade portion does not pass through the casing and cause damage to the aircraft carrying the engine.
There are various ways in which the problem of fan blade containment may be tackled. The most obvious way is to manufacture the fan casing from an alloy which is sufficiently strong and thick to provide the desired degree of containment. However this almost invariably results in a fan casing which is undesirably heavy. An alternative approach is to provide an alloy fan casing which is thin, and therefore light, and wind around it a strong fibrous material such as an aromatic polyamide. In the event of all or part of a fan blade becoming detached, it passes through the thin alloy casing but is contained by the fibrous material.
There is a danger that the detached fan blade or blade part could cut through part of the fibre wrap, thereby reducing its ability to provide effective containment. This problem is addressed in GB2159886B by the provision of patches of the fibrous material interposed between the wound fibrous material and the casing. In the event that a fan blade or blade portion becomes detached, it pierces and passes through the casing to engage some of the patches. The patches wrap around the leading regions of the fan blade or blade portion to define a pad which in turn protects the wound fibrous material from being cut by those leading regions.
It is important for the effective operation of fan blade containment systems of this kind that the patches of fibrous material are minimally constrained. However this can be difficult to achieve since they can be compressed by the wound fibrous material which surrounds them. However if they are not tightly held by the wound material, there is a danger that they could slip into undesirable positions. Moreover any slackness in the wound material could compromise its effectiveness in providing fan blade containment.
There is a further difficulty with such fan casing constructions in that, in the interest of lightness, the alloy part of the fan casing is made as thin as possible. This can lead to a lack of adjacent stiffness in the casing. The problem is particularly severe in the case of the more powerful ducted fan engines which have very large diameter fan casings. Any thickening of the casings to provide the necessary degree stiffness is likely to increase the weight of the casings to undesirable levels.